As widely differing technical systems are increasingly networked, there is a growing requirement for standardized structures in industry. In this context, it is also desirable to be able to couple any desired appliances locally. In order to provide open systems for networking, it is necessary to provide simple and cost-effective communication mechanisms which enable industrial appliances to have a networking capability. In particular, this requirement also exists in the context of the coupling of drive components, such as drive controllers, power sections and transmitters, for numerically controlled machine tools and robots, in which a number of interpolating axes must be operated synchronously.
In present-day high-performance drive systems, the interfaces to transmitters and power sections are in the form of analog signal interfaces. This, however, involves considerable restrictions relating to the spatial distribution capability, since the susceptibility to interference from EMC effects (EMC stands for electromagnetic compatibility) increases with the cable length. If the performance requirements are low, proprietary company serial digital transmission systems are generally used. Where the performance requirements are high, the communication between the drive controller and the movement controller is provided by proprietary-company serial data transmission systems.
Recently, with regard to the requirement to provide industrial appliances with a networking capability the ETHERNET (data transmission rate 10 Mbps), in particular the FAST ETHERNET (data transmission rate 100 Mbps—IEEE Standard 802.3-1998) data transmission technology which is known from office technology, has been becoming increasingly important. This is due to the fact that this development represents an undefined Standard with regard to compatibility and, furthermore, is available at low cost, since appropriate interface hardware is being produced in large quantities, owing to the widespread use in the field of personal computers. Furthermore, ETHERNET networks are already widely used in many organizations, so that a widely extended infrastructure can already be made use of. These arguments all favor the use of the ETHERNET in the field of automation technology as well. ETHERNET is generally used in the field of Local Area Networks LAN, with the most widely used transmission protocol being TCP/IP (Transmission Control Protocol/Internet Protocol).
Accordingly, the IEEE Standard 802.3, CSMA/CD (Carrier Sense Multiple Access/Collision Detect) is generally used as the access method. In this method, all the network subscribers have equal priority, and any network subscriber is allowed to send a message on the network, generally a bus system at any time. However, a problem occurs in this case when two or more subscribers are sending a message at the same time. In this situation, a collision is identified, and each subscriber involved is then assigned a waiting time, which is defined randomly, before another attempt to send the message is made. The term statistical access method is therefore used.
The requirements for the performance of communication systems for automation technology are particularly stringent, for example when coupling drive components. When interchanging data between transmitters, power sections and a drive controller, the data transmission time, which is included in the control loop as a dead time, is a particularly important parameter. The shorter this dead time, the better the dynamic response which can be achieved by the control system.
The connection between movement controllers and drive controllers is also sensitive to dead times, since a control loop is also closed via this connection. There is thus a problem in particular in the data transmission time for serial communication systems, which can be solved only by an appropriately fast system with a real-time capability, that is to say a deterministic system. However, the fact that communication using ETHERNET networks does not ensure a determined time response runs counter to their use for automation technology. The ETHERNET Standard therefore does not offer the technical preconditions for real-time communications.
IEEE Standard 802.3 defines a message frame which is not suitable for this purpose. Since, however, the components for physical ETHERNET signal transmission are independent of the Standard protocol form, the developer has freedom to choose the protocol form in which the data are to be transmitted. Only Layer 1 (the physical layer) is adopted from IEEE Standard 802.3.